Storage

• FC-0 Fibre Channel Physical Media
• FC-1 Fibre Channel Encode and Decode
• FC-2 Fibre Channel Framing and Flow Control
• FC-3 Fibre Channel Common Services
• FC-4 Fibre Channel Upper Level Protocol Mapping.

Fiber Optics

There are two basic types of optical fiber: Multimode Fiber (MMF) and Single-Mode Fiber (SMF). MMF has a larger core diameter of 50 μm or 62.5 μm (the latter was common for Fiber Distributed Data Interface [FDDI] and carries numerous modes of light through the waveguide). It’s generally less expensive than SMF, but its characteristics make it unsuitable for distances greater than several hundred meters. Because of this, MMF is generally used for short distance spans and is common for interconnecting SAN equipment within the data center.

SMF has a smaller core diameter of 9 μm and carries only a single mode of light through the waveguide. It’s better at retaining the fidelity of each light pulse over long distances and results in lower attenuation. SMF is always used for long-distance extension over optical networks and is often used even within the data center for FICON installations. Figure 1 describes various types of optical fiber and operating distances at different speeds.

There are several types of SMF, each with different characteristics that should be taken into consideration when a SAN extension solution is deployed. Non-Dispersion Shifted Fiber (NDSF) is the oldest type of fiber and was optimized for wavelengths operating at 1310 nm, but performed poorly in the 1550 nm range, limiting maximum transmission rate and distance. To address this problem, Dispersion Shifted Fiber (DSF) was introduced. DSF was optimized for 1550 nm, but introduced additional problems when deployed in Dense Wavelength Division Multiplexing (DWDM) environments. The most recent type of SMF, Non-Zero Dispersion Shifted Fiber (NZ-DSF) addresses the problems associated with the previous types and is the fiber of choice in new deployments.

As light travels through fiber, the intensity of the signal degrades; this is called attenuation. The three main transmission windows in which loss is minimal are in the 850, 1310 and 1550 nm ranges. Figure 2 lists common fiber types and the average optical loss incurred by distance.

Fiber Loss and Link Budgets

A key part of designing SANs over long-distance optical networks involves analyzing fiber loss and power budgets. The decibel (dB) unit of measurement describes loss mechanisms in the optical path of a fiber link. Decibel loss is usually determined by comparing the launch power of a device to the receive power. Launch and receive power are expressed as decibel milliwatt (dBm) units, which is the measure of signal power in relation to 1 mW.

The link power budget identifies how much attenuation can occur across a fiber span while sufficient output power for the receiver is maintained. It’s determined by finding the difference between “worst-case” launch power and receiver sensitivity. Transceiver and other optical equipment vendors typically provide these specifications for their equipment. A loss value of 0.5 dB can be used to approximate attenuation caused by a connector/patch panel. An additional 2 dB is subtracted as a safety margin:

Power Budget = (Worst Case Launch Power) – (Worst Case Receiver Sensitivity) + (Connector Attenuation)

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